The present invention relates generally to AM receivers and more specifically to a receiver capable of software implementation of varies elements in AM receivers.
Despite the fact that the AM receiver is almost a century old the software radio implementation is still cost prohibitive. Currently, AM receivers are implemented in hardware, employing AM/FM chips at record low cost. A simplified block diagram of a conventional AM receiver is illustrated in FIG. 1. However, in a multi protocol communication hand held device, the hardware implementation becomes less attractive due to extra cost and PC board area consumed. The most attractive alternative for multi protocol communication devices is provided by the Software Defined Radio (SDR). The SDR has the ability to execute multiple communication protocols in the same processor but requires a different RF front end for each of them. As the processor capabilities evolve to higher MIPS, the border between analog and digital components moves closer to the antenna.
The hardware architecture of the present AM receiver uses fewer hardware components and allows an efficient software implementation by reducing the MIPS required. It provides both flexibility and low power consumption. A group of the AM receiver functions, including filtering and demodulation, are implemented in software.
The amplitude modulation receiver includes an antenna for receiving a signal and an input filter connected to the antenna. A variable gain amplifier is connected to the input filter and is responsive to a gain control signal. An A/D converter is connected to the variable gain amplifier, is responsive to a sampling signal and provides a sampled digital signal. A D/A converter receives a demodulated digital signal and provides an analogue output signal. A controller receives and demodulates the sampled digital signal from the A/D converter, generates the gain control signal for the variable gain amplifier, generates the sampling signal for the A/D converter, and provides the demodulated signal to the D/A converter.
The controller may be a multi-thread processor performing the demodulation and signal generation tasks in parallel.
The controller provides a sampling signal to a phase locked loop whose output provides the sampling signal to the A/D converter. The sampling signal from the controller is provided to a voltage control oscillator of the phase locked loop. The controller controls the generation of a variable sampling signal whose rate is coherent with the carrier frequency of the received signal.
The demodulator of the AM receiver is implemented in software which includes an input filter for filtering an input signal; a decimator and integrator demodulator for demodulating the filtered input signal; and an output filter for filtering the demodulated signal. The demodulator and the input filter are tuned to the carrier frequency of the input signal and the output filter is tuned to the decimated carrier frequency of the input signal. Coefficients of the filters for each carrier frequency are stored in the demodulator. The filtered signal is multiplied by a demodulation signal at a carrier frequency, integrated over a carrier cycle period and then decimated.
These and other aspects of the present invention will become apparent from the following detailed description of the invention, when considered in conjunction with accompanying drawings.